Found this and it seems to confirm my earlier investigations - RAID 10 seems to be better for performance if you can afford the extra disks.
I've also read now that it rebuilds a lot faster in the event of failure and can tolerate 2 disk failures in a 4 disk array, providing you are lucky and don't have them both fail in the same striping array. There also seems to be less of a risk of data loss in case of failed consistency checks.
more info here
I've gone for RAID 10 now and it seems to have improved performance a bit.